Wave signaling system



April 13, 1937. c. K. H"UX TAB"L E WAVE SIGNALING SYSTEM Fiied March 22, i935 s sheets-sheet 1 INVENTOR am: /r. HaxrABLE Patented Apr. 13, 1937 PATENT OFFICE WAVE SIGNALING SYSTEM Clyde K. Huxtable, Flushing, N. Y., assignor to Hazelt ine Corporation, a corporation of Dela ware Application March 22,

Claims.

amplified carrier at substantially constant amplitrade for a wide range of variation in the carrier amplitude impressed on the amplifier.

Systems of automatic amplification or volume control have been devised wherein the amplified carrier signals are directly applied to a rectifier for deriving a unidirectional biasing voltage which is applied to the amplifier control electrodes to reduce the amplification in inverse relation to the impressed carrier wave amplitude. Such systems are limited as to the closeness of regulation afiorded in that there can be no increase in unidirectional biasing voltage without a corresponding increase in amplitude of the amplified carrier.

In United States Letters Patent No. 2,050,679, granted August 11, 1936, upon the application of Harold A. Wheeler, there is described an improved form of automatic volume control in which the output of the signal amplifier is coupled to a rectifier through an auxiliary amplifier. This system is featured in simultaneous automatic control of both the signal and auxiliary amplifiers, the automatic control being reversely applied to the auxiliary amplifier as compared to the signal amplifier and such as to increase the amplification of the former and simultaneously to reduce the amplification of the latter in .response to an increase in signal intensity impressed on the signal amplifier. Exceedingly close regulation is obtainable with this type of system, because variations in the output of the signal amplifier are magnified in the auxiliary amplifier prior to rectification. Therefore a relatively small change in output of the signal amplifier produces a relatively large change in unidirectional voltage derived from the rectifier and applied to the signal amplifier to compensate the change.

The present invention employs the principle of reverse automatic volume control of the mentioned Wheeler application and to this end couples the output of the signal amplifier to the rectifier through a reversely controlled auxiliary amplifier. Whereas, however, the Wheeler system achieves reverse automatic control of the auxiliary amplifier through variations in the cathode voltage of a controlled signal amplifier tube, in accordance with the present invention,

1935, Serial No. 12,359

the auxiliary amplification is reversely controlled directly by means of a unidirectional voltage derived from the rectifier.

The present invention employs for coupling the output of the signal amplifier to the biasing rec- 5 tifier, an auxiliary vacuum tube amplifier such that a negative increase in biasing voltage applied to a control grid thereof causes signals to be amplified increasingly. The increase in amplification of the auxiliary amplifier does not exceed the accompanying decrease in amplification cf the signal amplifier, and is so proportioned that the output of the signal amplifier is held more nearly constant than the output of the auxiliary amplifier. The ratio of unidirectional biasing voltage derived from the rectifier to the outputvoltage of the signal amplifier is thus increased with increase of signal voltage impressed on the amplifier.

Utilization of an auxiliary amplifier, the amplification of which increases with negative increase in biasing voltagev applied to its control grid, greatly simplifies the automatic regulation in that the requisite control grid bias for the auxiliary amplifier may be obtained by simply tapping to the same unidirectional rectifier voltage which biases the control grids of the signal amplifier. The fraction of the total biasing voltage derived from the rectifier which is employed to bias the auxiliary amplifier may be so selected as to hold the output or the signal amplifier practically constant Within the region of automatic regulation.

For securing increased amplificationwith negative increase in biasing voltage the auxiliary amplifier comprises preferably a pentode tube containing in its plate lead a high resistance such as in the absence of signals, to reduce its plate voltage and hence its mutual conductance nearly to zero. Thus a negative increase in its grid bias causes the average plate current to decreaseand the average plate voltage to rise producing thereby an increased mutual conductance with consequent increased amplification.

The rate of increase in amplification, which is practically negligible below a fairly well defined negative grid biasing voltage, rises rapidly and substantially uniformly thereafter to a high maximum level of amplification. There is preferably applied to the auxiliary amplifier cathode, a fixed positive bias such as initially to adjust the operating point close to the substantially linear portion of the grid bias-amplification characteristic. :The fraction of the total unidirectional biasing voltage which is applied to the grid of the auxiliary amplifier is so selected that the rate of increase in control biasing voltage substantially coincides with the linear portion of the amplifier characteristic. With these adjustments 5 the automatic control will be substantially nil for signal voltage below a preselected threshold value applied to the signal amplifier, but will be such as to hold the output of the signal amplifier substantially constant above the threshold level at which the automatic control takes hold.

In the drawings: 7

Fig. 1 is a diagrammatic showing of a superheterodyne radio receiving system having associated therewith a system of automatic volume control in accordance with the present invention in which separate vacuum tubes are employed to provide auxiliary amplification and rectification respectively.

Figs. 2-4 inclusive are graphs illustrating the adjusting and operation of the system of Fig. 1.

Figs. 5 and 6 show modified forms of the Fig. 1 auxiliary amplifier and rectifier circuit.

Fig. 7 is a circuit diagram of an embodiment of the automatic volume control in accordance with the present invention wherein a single vacuum tube serves as a combined auxiliary amplifier and rectifier.

In the superheterodyne receiving system of Fig. 1, a primary coil of a double tuned selector 2, 3o completes a circuit from antenna 3 to ground 4, for selectively transferring modulated carrier wave signals from the antenna to a stage of radiofrequency amplification, tube 5, the output of which is applied through a condenser tuned selector 6 to the control grid of a modulating element, tube '5. A source of heterodyne oscillations comprising tube 8 having its electrodes regeneratively coupled by a tuned impedance 9, applies said oscillations to the input of the modu- 40 lating element by Way of a transformer Hi, having a secondary coil it included in lead I2 connecting the cathode of tube '1 to ground through a biasing impedance lit.

The modulated or intermediate frequency out- 5 put of tube is selectively impressed on a stage of intermediate frequency amplification, tube I4, through an expanding selector l5, consisting of a transformer I6 having condenser tuned primary and secondary coils ll and i8, respectively rela- 50 tively displaceable axially, as shown by the double ended arrow, for varying the magnetic coupling therebetween and thereby the Width of the resonance band in the manner described in United States Letters Patent No. 2,024,017, granted De- 55 cember 10, 1935, upon the application of Harold A. Wheeler and Nelson P. Case.

The output of tube i4 is impressed through an expanding selector l9, similar to l5, on the signal control grid 29 of a multi-electrode tube 2| of type 60 2131 operating as a combined stage of intermediate frequency amplification and signal rectifier. Amplification at the intermediate frequency occurs in consequence of the action of the control grid 29, on the electron stream passing 65 between cathode 22 and plate 23. The amplified signals present in the plate lead 24 are applied through transformer 25, tuned by condenser 28 to theintermediate frequency, between the oathode 22 and the rectifier anodes 21 connected to- 7 gether externally of the tube. A condenser 28 lay-passes the high and intermediate frequency components around a resistance 29, for selectively applying over lead 30 connected to the in putof the audio-frequency amplifier 3|, the

75 audio-frequency voltage developed across resistance 29. To this end the cathode 22 is grounded through a biasing impedance 32 and the cathode system of the audio amplifier is grounded at 33. A loud speaker 34 or other desired translating device, responds to the output of amplifier 3|.

Automatic control of the signals relayed from antenna 3 to loud speaker 34 is effected by the auxiliary means within rectangle 40. The signal carrier amplifier output voltage Es developed across the secondary transformer coil of selector l9 grounded at 4|, is applied over a connection 42 and through a blocking condenser 43 to the signal control grid 44 of a pentode tube 45.

Pentode tube 45 is an intermediate frequency amplifier whose output is coupled to a diode rectifier, tube 46, through a transformer 41 comprising condenser tuned primary and secondary coils coupled broadly to tune to the intermediate frequency. The rectifier circuit is completed from its grounded cathode 48 to its anode 49 through serially connected resistors 50 and 5| shunted by a condenser 52 so small as to have negligible by-passing effect at audio-frequencies.

The rectified voltage Er developed across resistors 50 and 5|, derived from the signal carrier amplifier output voltage Es, is applied over connection 53 and through the transformer secondaries of selectors 2, 6 and I5 to the signal control grids respectively of tubes 5, and M, thereby to reduce the amplification in the mentioned radio and intermediate frequency stages in the usual manner. The series resistors 54 and the grounded condensers 55 remove the modulation components of Er.

The fraction E0, across resistance 5|, of the total biasing voltage Er, is applied over connection 53 and through leak resistor 51 to the control grid 44 of the auxiliary amplifier tube 45. The grounded condenser 58 removes the modulation components of Ec.

The biasing voltage EC increases the amplification of the auxiliary amplifier tube 45, simultaneously with decrease in the amplification of tubes 5, l and I4, produced by voltage Er. It is the purpose of the biasing voltage Ec, termed a reverse automatic volume control, to maintain Es nearly constant regardless of the intensity of the antenna input voltage Ea above a threshold value. It is the additional increase in amplification of the auxiliary amplifier tube 45, produced by the biasing voltage Ec, which permits the rectified voltage Er to increase without a proportionate increase in E5.

The complete circuit is considered to employ reverse automatic volume control, because the rectified voltage is used to vary the amplification of the signal carrier amplifier and simultaneously to vary reversely the amplification of the auxiliary amplifier. I

In another sense the auxiliary amplifier tube may be considered to operate with reverse automatic volume control in that its output is permitted to vary without a corresponding change in its input, as contrasted to an ordinary automatically controlled amplifier whose input is permitted to vary without a corresponding change in its output.

The negative bias E0 applied to tube 45 is caused to produce an increasing amplification in the following manner: A high resistance 59, bypassed to ground by condenser 65, is included in series with the grounded plate battery 6| in the plate lead 62 of tube 45. Resistance 59 is such that the plate current is sufficient for small values of grid bias Ec applied to tube 45 to reduce negative. with respect to ground the average voltage at the plate 63 nearly to zero. I hen the average plate voltage is zero the mutual conductance and ,u. of tube 45 are zero.

Curve I of Fig. 2 represents for a given value of Es the rectified voltage Er as a function of the total grid bias applied to grid 44, and is proportional. to the amplification of tube 45. The amplification is nearly zero when the grid bias is less than four volts for the particular tube 45 of curve I00. As the grid bias is increased negatively beyond four volts, the average plate current in-tube 45 decreases and the average plate voltage rises correspondingly. Thus the mutual conductance and ,1. of the tube increase. As in dicated by curve I00, the maximum value of amplification is secured at a grid bias of eight volts. Thereafter while the plate voltage continues to rise, due to further negative increase of the biasing voltage, the emission is limited by the negative voltage on the grid and the amplification is againreduced. Thus there is a region from 4to' -8 volts inwhich a more negative grid bias produces amplification increasing at a high and substantially constant rate. It is within this region that the tube is operated. Er is not required to exceed the value shown for Er (max) of Fig. 2.

A fixed positive voltage Ed is applied between the cathode 64 of tube 45 and ground. When no signal is being rectified by tube 46, the grid 44 of tube 45 is at ground potential, and the grid bias of the tube is equal to Ed. This voltage is indicated in Fig. 2. In the presence of a signal the rectified voltage Ec carries the grid 44 more the total grid bias. The ratio of resistors 50 and determines the relationship between E]: and Ec. This proportionality is shown by the linear graph IOI of Fig. 2.

.Since the rectified voltage Er depends; on the grid bias of 44 according to curve I00, Fig. 2, and since the bias of 44 is determined by this same rectified voltage according to curve IOI, it is evident that equilibrium is reached at any point of intersection of curves I00 and I0l. Over the region in which the curves I00 and IOI coincide, Er may vary with no change in Es since curve I00 represents a condition of constant Es.

Within these limits an increase in the antennainput voltage Ea will tend to cause an increase in Es. However, an increase in Es instantaneously produces a proportional rise in Er and Es.

Es immediately increases the auxiliary amplification of tube 45 producing an additional rise in Er. This has the effect of reducing Es to its original value, and the circuit reaches equilibrium again at the same value of Es but at increased values of Er and Ec. In a circuit which does not embody the increased auxiliary amplification of tube 45, Es is never restored to its original value but to a value somewhat higher.

In Fig. 3, curve I03 represents the variation in the signal carrier amplifier output voltage Es as a function of the rectified voltage Er. The horizontal dotted line I02 shows the ideal condition where Es is constant regardless of Er. Curve I03 shows that the auxiliary amplifier and rectifier circuit in accordance with thepresent invention provides. operation in, accordance with the ideal except for exceedingly small voltages Es. The superior operation achieved by the present invention is apparent from a comparison of curve I03 with curve I04, the latter showing the variation present in prior types of control circuits where the -rectified voltage varies directly withEs.

and increases I Fig. 4 represents the resultant variation of Es with antenna inputvoltage Ea, and illustrates the improvement secured by the automatic con trol systems of the present invention. Curve I05 is the ideal condition in which Es is uniform above a threshold value Es. Curve I06 shows the response of the present circuit. Curve I 01 shows the response of a circuit corresponding to curve I04 of Fig. 3.

With the circuit of the present invention, the slope of curves- I03 and I06, Figs. 3 and 4, may be made negative if desired over a portion of their variation. This occurs, referring to Fig. 2, if there is a region in which the slope of curve I00 is greater than that of curve IN.

The failure of the Fig. 1 control circuit to attain the ideal for small voltages Es resides in the difference between curves I00 and IM of Fig. 2. Methods of reducing this difference may take the form of either (1) causing curve I00 to become more rectilinear, or (2) of varying curve IOI from a straight line to a curve more nearly coinciding with curve I00. Examples of these methods respectively are (l) applying an initial negative bias to the anode of rectifier 46; and (2) employing a non-linear resistive device, such as a sec ond diode tube, in place of resistance 5|, Fig. 1.

Fig. 5 illustrates within rectangle 40 the corresponding portion of the Fig. 1 circuit as modified by inclusion of a battery serving to apply a permanent bias to rectifier 46. The principal effect of the bias provided by battery 10, Fig. 5, is to move curve I00, Fig. 2, downward by an amount equal to the rectifierbias. This will require a larger value of Es to secure the necessary Er (max). In this way the lower curved portion of I00 is reduced and curves I00 and IM can be made to coincide over a greater proportion of their length; Hence curve I03, Fig. 3, will more nearly approach the horizontal. It will also intersect the Es ordinate at a value of Es greater than zero. This method has the disadvantage in that with an initial negative bias on the rectifier anode the average rectified voltage is not equal to the average carrier voltage in the presence of large percentages of modulation.

More nearly ideal response may also be secured by substituting for the resistor 5i, Fig. 1, a nonlinear device, such as a diode, across which the voltage drop varies according to a non-linear function of the current. Fig. 6 shows the portion within-rectangle 40 of the Fig. 1 circuit as modified to include this feature. In Fig. 6, diode II replaces resistance 5| of Fig. 1. In other respects the circuit portions 40 of Figs. 1 and 6 are identical.

By properly choosing the current'voltage function of device II, curve IOI, Fig. 2 (which'represents this function) may be made more nearly to coincide with curve I00 over the range of Er from zero'to Er (max.). When this is done, curve I03, Fig. 3, is more nearly horizontah Fig. '7 shows a circuit diagram of an automatic volume control circuit in accordance with the present invention, in which the auxiliary amplifier and rectifier electrodes are sealed within a single envelope. The control system of Fig. -7 may be substituted for control system 40 in Fig. :1 by breaking connections 42 and 53 of Fig. l and connecting them to leads 42 and .53 respectively of Fig. 7.

In Fig. 7, tube 75 of type 231, consists of a pair of diodes 10 and apentode amplifier witha common cathode. The amplifier portion of the .tube includessthe cathode 111. in ut materia (993- trol grid 18, inner screen 19, outer screen and the plate 8|. The rectifier comprises the cathode TI and the two anodes 1B which are strapped together external to the tube as shown.

The signal carrier amplifier output Es, Fig. 1, is applied over connection 42 through blocking condenser 43 to the signal control grid 18 of the pentode amplifier. The signal is amplified at intermediate frequency in the pentode section of the tube, the amplified signals present in the plate lead 82 being impressed on the diode rectifier through the broadly tuned selector 83, comprising condenser tuned primary and secondary transformer coils, the one included in the plate lead 82 and the other connected between the rectifier anodes l6 and cathode l1 through a high frequency by-pass condenser 84 shunting a resistive network 85. The plate BI is energized from a source 86 through a high resistor 81 such as normally to maintain the plate 8| at substantially zero potential to provide the reverse gain variation with grid potential in accordance with curve H30, Fig. 2.

Initial potentials are applied to the grid 18, cathode Ti, and anodes 16, by means of the bridge network comprising negatively grounded battery 88 shunted to ground by a pair of resistance paths consisting respectively of the serially connected resistors 89, 9B and 9|, D2, 93. The cathode TI is connected between the resistors 89 and 90 which form a relatively low resistance path to ground. Resistors 9!, 92, 93 form a relatively high resistance path to ground. The anodes It are returned through the selector 83 secondary coil to the point 94 between resistors 9| and 92. The resistors are so proportioned that the anodes T6 are initially at the same potential as the oathode TI. The control grid 18 is connected through a leak resistance 95 to the junction of resistors 92 and 93. Resistor 93 is relatively small in comparison with 92 and 9! in the ratio approximately of 1:10:10, and thus the grid 18 is initially nearly at ground potential.

In the presence of a signal, a rectified current fiows through the diode it, 11 and divides between the paths including resistors iill, 93, 92 and resistors 89, 9|, this current flowing in a direction to decrease the current in 92, 93 and to increase the current in 9 l. The result is that point 96 becomes less positive and may become negative with respect to ground. The voltage at the junction of 92, 93 follows the voltage of point 94 and is related to it in the ratio of the value of resistance 93 to that of the sum of resistances 92 and 93.

Any decrease in the voltage at the point 94 effectively increases by way of connection 53 the grid bias on tubes 5, 'l and i4, and reduces their amplification. The decrease in voltage at 9d is accompanied by a proportionate decrease in voltage at the junction of 92 and 93, which effectively increases the amplification in the pentode section of tube 15. The operation is then essentially that described with reference to the control system 40 of Fig. 1.

The purpose of the resistance network 85 is to secure an initial bias on grid 18 without an initial bias on the diode section 16, ll. Such a diode bias is undesirable because it causes the amount of automatic control bias to be affected by the percentage modulation of the carrier.

Referring again to Fig. 1, the double tuned intermediate frequency selectors l5 and I9 are constructed to have adjustable couplings between their respective primary and secondary coils as indicated. The selectivity of the receiver may be adjusted at will by changing these couplings. Adjustment is effected in the manner described in the U. S. Patent No. 2,024,017, referred to, by means of a unitary control 96 adapted to alter simultaneously and to like extent the axial separations of the primary and secondary coils in the mentioned selectors. The preceding selectors 2 and 5 are relatively broad so that the selectivity of the receiver is chiefly dependent upon the two variable selectors l5 and I9.

When the coupling is adjusted to its maximum value, as by abutment of the unitary control 96 with a stop 91, the resonance curve of the receiver from the antenna to the grid of tube 2| has a double peak because of the over-optimum coupling in the variable selectors l5 and I9. Single-tuned selector 25 is sharply tuned to the intermediate frequency and its response is such as to compensate for the double peaks produced by selectors I5 and I9. This arrangement has the advantage of securing in the carrier channel a fiat response characteristic over the desired frequency band.

The signal level at the input to tube 2| is held constant by the action of the automatic control as the user tunes through a station. The presence of the single-tuned circuit 25, following the automatic volume control in the receiving system, causes an artificial maximum in the intensity of the reproduced signal when the carrier is tuned to the intermediate frequency. This aids the user in tuning the receiver correctly, and reduces the intensity of harsh and unpleasant signals which are received when detuned slightly.

I claim:

1. In a signaling system, a signal-translating channel including an implifier producing an amplified signal output, auxiliary means external to said signal-translating channel and responsive to said amplified signal output for producing a unidirectional voltage, means for controlling the amplification in said signal amplifier in accordance with said unidirectional voltage, and means for simultaneously directly applying unidirectional voltage produced by said auxiliary means to a control element of said auxiliary means reversely to control the responsiveness of said auxiliary means, whereby the ratio of said unidirectional voltage to said amplified signal output is increased when the signal input applied to said amplifier increases.

2. In a modulated carrier frequency signaling system, a signal-translating channel including a carrier amplifier producing an amplified carrier output, means external to said signal-translating channel responsive to said amplified carrier output for producing a unidirectional voltage, said means including an auxiliary amplifier and a rectifier, means for reducing the amplification in said carrier amplifier in accordance with said unidirectional voltage, and means for simultaneously directly applying unidirectional voltage produced by said rectifier to a control element of said auxiliary amplifier to increase the amplification in said auxiliary amplifier, whereby a change in the intensity of input voltage to said carrier amplifier is caused to produce a greater ratio of change in said unidirectional voltage than in said amplified carrier output.

3. In a signaling system, a signal-translating channel including a signal amplifier, means external to said signal-translating channel including an auxiliary amplifier and a rectifier for producing a unidirectional voltage in response to the output of said signal amplifier, said auxiliary amplifier comprising a vacuum tube having an output electrode, means for reducing the amplification in said signal amplifier and simultaneously increasing the amplification in said auxiliary amplifier, by causing the average voltage of said output electrode to increase, in accordance with said unidirectional voltage.

4. In a signaling system, a signal amplifier producing an amplified signal output, an auxiliary pentode amplifier coupling said output to a rectifier producing a rectified signal voltage, said pentode amplifier having in its plate lead a high resistance which reduces its average plate voltage nearly to zero, means for reducing the amplification in said signal amplifier in accordance with said rectified voltage and simultaneously to increasing the amplification in said pentode amplifier, by making its control grid more negative and thereby increasing its plate voltage, in accordance with said rectified voltage.

5. In a signaling system, a first amplifier for amplifying the signal, a second amplifier for further amplifying the signal, a rectifier responsive to the output of the second amplifier, a connection from said rectifier to said first amplifier for automatically reducing the amplification therein, and a connection from said rectifier to said second amplifier for automatically increasing the amplification therein, said increase in amplification in said second amplifier being less than said reduction in amplification in said first amplifier and being proportioned to hold the output of the first amplifier more nearly constant than the output of the second amplifier.

6. In a signaling system, a signal-translating channel including a signal amplifier, means external to said signal-translating channel including an auxiliary amplifier and a rectifier for producing an amplified unidirectional voltage in response to the output of said signal amplifier, means for controlling the amplification in said signal amplifier in accordance with said unidirectional voltage, and means for simultaneously directly utilizing said unidirectional voltage to reversely control the amplification in said auxiliary amplifier, whereby in response to a variation of input to said signal amplifier said auxiliary amplification is caused to vary in substantially the same ratio as said unidirectional voltage.

7. In a signaling system, a signal amplifier pro ducing an amplified signal output, a pentode amplifier coupling said output to a circuit including a rectifier for producing a unidirectional voltage dependent on said output and on the amplification in said pentode amplifier, a connection from a point in said rectifier circuit to a control element of said signal amplifier for controlling said output in response to said unidirectional voltage, a connection from a point in said rectifier circuit to the control grid of said pentode amplifier for automatically adjusting the control grid bias, a high resistance in the plate lead of said pentode amplifier proportioned to cause said amplification to increase in predetermined relation to increase of control grid bias, whereby in response to a variation of input to said signal amplifier said unidirectional voltage is caused to vary with respect to said control grid bias substantially in accordance with said predetermined relation.

8. In a signaling system, a signal amplifier producing an amplified signal output, a pentode amplifier coupling said output to a circuit including a rectifier for producing a unidirectional voltage dependent on said output and on the amplification in said pentode amplifier, a connection from a point in said rectifier circuit to a control element of said signal amplifier for controlling said output in response to said unidirectional voltage, a connection from a point in said rectifier circuit to the control grid of said pentode amplifier for automatically adjusting the control grid bias, a high resistance in the plate lead of said pentode amplifier proportioned to cause said amplification to increase in predetermined relation to increase of control grid bias, and means for preventing said rectifier from responding until the output from the pentode amplifier exceeds a predetermined value, whereby in response to a variation of input to said signal amplifier said unidirectional voltage is caused to vary with respect to said control grid bias substantially in accordance with said predetermined relation. i

9. In a signaling system, a signal amplifier producing an amplified signal output, a pentode amplifier coupling said output to a circuit including a rectifier for producing a unidirectional voltage dependent on said output and on the amplification in said pentode amplifier, a non-linear device in the rectifier circuit, a connection from a point in said rectifier circuit to a control element of said signal amplifier for controlling said output in response to said unidirectional voltage, a connection from said non-linear device to the control grid of said pentode amplifier for automatically adjusting the control grid bias according to a non-linear function of said unidirectional voltage, a high resistance in the plate lead of said pentode amplifier proportioned to cause said amplification to increase in predetermined non-linear relation to increase of control grid bias, whereby in response to a variation of input to said signal amplifier said unidirectional voltage is caused to vary with respect to said control grid bias substantially in accordance with said predetermined nonlinear relation.

10. In a signaling system, a signal-translating channel including an amplifier producing an amplified signal output, means external to said signal-translating channel including auxiliary means responsive to said amplified signal output for producing a unidirectional voltage, means for reducing the amplification in said amplifier in ac cordance with said unidirectional voltage, and a direct current connection from the output of said auxiliary means to a control element of said auxiliary means for increasing the responsiveness thereof in accordance with said unidirectional voltage, said increase in the responsiveness of said auxiliary means being less than said reduction in amplification in said amplifier and being proportioned to hold the output of the first said amplifier more nearly constant than the responsiveness of said auxiliary means.

CLYDE K. HUXTABLE. 

